Q: A 5 kg object is acted upon by a net force of 20 N. What is its acceleration?

$$a = \frac{F_{\text{net}}}{m} = \frac{20}{5} = 4\ \mathrm{m/s^2$}$$

Q: A person stands on a scale in an elevator that is accelerating upward at $$2\ \mathrm{m/s^2$}. If $$the person's mass is $$70\ \mathrm{kg}$$, what is the reading on the scale?

$$F = m(g + a) = 70 \times (9.8 + 2) = 826\ \mathrm{N}$$

Question 1

Which of the following statements best describes Newton's First Law of Motion?

Accepted Answer

An object will remain at rest or move with constant velocity unless acted upon by a net external force.

Question 2

A hockey puck slides across a frictionless ice surface with constant velocity. Which statement is true about the forces acting on the puck?

Accepted Answer

The net external force acting on the puck is zero.

Question 3

A 5 kg object is acted upon by a net force of 20 N. What is its acceleration?

Accepted Answer

$$a = \frac{F_{	ext{net}}}{m} = \frac{20}{5} = 4\ \mathrm{m/s^2$}$$

Question 4

Which of the following is NOT a contact force?

Accepted Answer

Gravitational force

Question 5

A person stands on a scale in an elevator that is accelerating upward at $$2\ \mathrm{m/s^2$}. If $$the person's mass is $$70\ \mathrm{kg}$$, what is the reading on the scale?

Accepted Answer

$$F = m(g + a) = 70 	imes (9.8 + 2) = 826\ \mathrm{N}$$

Question 6

Which of the following best explains why astronauts in orbit feel 'weightless'?

Accepted Answer

They are in continuous free fall, so both they and their surroundings accelerate at the same rate.

Question 7

A box is pulled across a horizontal surface by a rope with tension $$T. If $$the box moves at constant velocity, which statement is true?

Accepted Answer

The tension force is equal in magnitude to the friction force.

Question 8

A 40 kg crate is pushed with a constant force of 20 N on a frictionless floor. What is its acceleration?

Accepted Answer

$$a = \frac{F}{m} = \frac{20}{40} = 0.5\ \mathrm{m/s^2$}$$

Question 9

Which of the following is a correct statement of Newton's Third Law?

Accepted Answer

If object A exerts a force on object B, then object B exerts an equal and opposite force on object A.

Question 10

A car suddenly stops and crash test dummies move forward. Which law explains this phenomenon?

Accepted Answer

Newton's First Law (Law of Inertia)

Question 11

A block is suspended by a rope. If the block is at rest, what is the relationship between the tension in the rope and the weight of the block?

Accepted Answer

The tension equals the weight: $$T = mg$$

Question 12

Which of the following is true about the net force acting on an object in equilibrium?

Accepted Answer

The net force is zero: $$\sum \vec{F} = 0$$

Question 13

A 2 kg object is moving in a circle of radius 3 m at a constant speed of 4 m/s. What is the magnitude of the net force acting on the object?

Accepted Answer

$$F = m \frac{v^2$}{r} = 2 	imes \frac{16}{3} = 10.67\ \mathrm{N}$$

Question 14

Which of the following is a scalar quantity?

Accepted Answer

Mass

Question 15

A person pushes backward on the ground while walking. According to Newton's Third Law, what force propels the person forward?

Accepted Answer

The ground pushes forward on the person with equal magnitude.

Question 16

A 10 kg object is on the Moon, where $$g = 1.6\ \mathrm{m/s^2$}. $$What is its weight on the Moon?

Accepted Answer

$$W = m g = 10 	imes 1.6 = 16\ \mathrm{N}$$

Question 17

Which of the following is true about action-reaction pairs according to Newton's Third Law?

Accepted Answer

They act on different objects and are of the same type.

Question 18

A block is pulled by a rope with tension $$T$$ and moves at constant velocity. If the block and rope are considered as a single system, what happens to the internal tension forces?

Accepted Answer

The internal tension forces cancel out and do not affect the net force on the system.

Newton’s Laws of Motion – Study Notes

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